We still have no interstellar pen pals. But Dr. Drake's Project Ozma foreshadowed more than 60 subsequent programs in the ongoing Search for Extraterrestrial Intelligence (SETI).

Carried on largely by the United States and the former Soviet Union with some other nations participating, SETI has advanced the art of detecting and processing faint radio signals. It has provided a unifying theme for K-12 and college-level studies that explore the astronomy, biology, chemistry, and physics underlying organic life.

It also has pioneered the use of personal computers to process floods of data. Something like a million volunteers have joined the SETI@home program of the University of California at Berkeley. Free software receives raw data from Project Serendip - a receiver at the Arecibo Radio Observatory in Puerto Rico - and sends back results.

These are practical achievements that Drake and other SETI enthusiasts can celebrate April 8, International Astronomy Day. Their work is focused as much on accomplishments on Earth as on detecting alien civilizations. Yet what physicist Philip Morrison calls our species' "most challenging exploration" inspires their enterprise. Writing in the journal Nature in 1959, Dr. Morrison and Giuseppe Cocconi, both then at Cornell University, detailed the possibility of detecting alien radio signals. They echoed the vision of radio pioneer Guglielmo Marconi who, in 1919, said the reach of radio "makes me hope for a big thing in the future ... communication with intelligences on other stars."

Critics devalue that hope, judging intelligent life a very rare phenomenon. Politicians have repeatedly ridiculed it. Yet when the US Congress scuttled NASA's SETI program, it was reborn in programs of the privately funded SETI Institute in Mountain View, Calif., and of The Planetary Society. The institute now is developing the most powerful SETI radio telescope yet. Inexpensive dishes will cover a hectare (2.47 acres) in a linked array. They can observe several stars simultaneously - covering a million targets in a few years.

"In science, you have to advance by climbing the ladder one step at a time," says Drake. But 40 years ago, we crossed what he calls the crucial "threshold where we could detect [alien] civilizations."